1. Field of the Invention
This invention relates to a method of producing optical fiber cable having one or more optical fibers surrounded by a sheath.
2. Description of the Prior Art
In the manufacture of optical fiber cables, one or more optical fibers are typically surrounded by sheathing and strength members, as required, to enable manufacture, installation, and operation of the cable while preventing damage to the optical fibers. For example, steps are typically taken to prevent buckling of the optical fibers due to compressive longitudinal forces during manufacturing or subsequently during the lifetime of the cable, in order to prevent significant microbending losses from occurring. Microbending losses increase the attenuation of the optical radiation in the fibers, limiting the length of transmission in a fiber; see, for example, Gardner, "Microbending Loss in Optical Fibers", Bell System Technical Journal, Vol. 54, No. 2, pages 457-465, February 1975. One type of cable construction is shown in U.S. Pat. No. 4,241,979, coassigned with the present invention. In that cable, microbending losses are substantially reduced or prevented by decoupling the optical fibers from the surrounding cable. That is, the optical fibers, typically formed into ribbons, are placed inside an inner sheath with sufficient space between the ribbons and the sheath to prevent longitudinal forces from the sheath from substantially acting on the fibers. In addition, the ribbons are given a slight twist to reduce bending stresses.
To prevent the entry of water into cables, including optical fiber cables, a filling compound can be used to fill the spaces surrounding the fibers inside the cable; see, for example, U.S. Pat. No. 4,259,540, coassigned with the present invention. The resultant cable is often referred to as a "filled cable". The filling compound is typically in the form of a gel which allows for flexibility of the cable while preventing significant water entry. However, when used with optical fiber cables, filling compounds may couple the optical fibers to the surrounding sheath so that longitudinal stresses in the sheath are transmitted to some degree to the fibers. Therefore, if contraction of the sheath occurs during the manufacturing process, or during the service life of the cable, filled cables can produce an unsatisfactorily high degree of microbending losses for the optical fibers therein. Such contraction is more likely to be significant when the sheath is made of polymeric material, as is typical in the art.
In the past, to prevent such stresses being transmitted to the optical fibers, strength members (for example, longitudinal steel wires) have been included in filled optical fiber cables to prevent substantial contraction from occurring during manufacture. Such strength members are typically located within the center of the cable, or within a cluster of optical fibers, or embedded in the polymer inner sheath. However, the above method has significant drawbacks. For example, in many cases it is desirable to avoid the added complexity and weight that reinforcing members require. Further, such members complicate the design of gripping hardware required for installation. Therefore, it is desirable to find an alternate method of making an optical fiber cable having a sheath surrounding, and coupled to, one or more optical fibers that eliminates or substantially reduces microbending losses in the fibers.